1. Field of the Invention
The invention pertains to the field of engine timing drive assemblies. More particularly, the invention pertains to a system and method for assembling an engine timing drive.
2. Description of Related Art
Automobile engine timing drive components are commonly shipped as a kit. The components are sometimes packaged together in a relationship of how they are to be placed in the engine. The sprockets and chain may be placed in a molded tray in a timed relationship along with the pre-assembled mechanical tensioner. At the engine assembly plant, a chain and its sprockets may be mounted on the respective shafts as a timed unit, but the tensioners for the chain are individually mounted usually before the chain and sprockets are mounted.
In U.S. Patent Application Publication No. 2005/0130776 “A CASSETTE FOR SECURING SHIPPING AND ASSEMBLY OF A CAMSHAFT DRIVE AND TIMING SYSTEM”, the cassette is for a guide and a tensioner attached to a tensioner bracket, a chain, a camshaft sprocket, and a crankshaft sprocket. The cassette includes a body defining clearance holes and shoulder guides for placement of the camshaft sprocket, the crankshaft sprocket, and the chain. The cassette also includes at least one fastener engaging the body of the cassette to the tensioner bracket. When the body of the cassette is attached to the tensioner, the chain runs around the camshaft sprocket and the crankshaft sprocket. The guide and the tensioner are positioned in the same relationship and position on the chain as the components are installed on the engine. All components are pivotally placed for installation on the engine.
Referring to FIG. 1 through FIG. 3, the prior art cassette 10 has a loop 12 for storage, shipping, and handling purposes. The cassette 10 also contains a camshaft shield 14 which offers protection to the camshaft sprocket 16 from damage by an adjacent cassette during shipping. The camshaft shield 14 also protects from sagging of the camshaft sprocket 16 and traps fasteners used to secure the camshaft sprocket 16 to its respective shaft on the engine. The cassette 10 contains three shoulder guides (not shown) visible from the back side for the crankshaft sprocket 18, camshaft sprocket 16, and the chain 20. The cassette 10 has two receiving holes 22 slightly off center. The tensioner bracket 24 is visible through the receiving holes 22 in FIG. 1. Adjacent to the receiving holes 22 is a handle 26 which turns a tab (not shown) on the back of the tensioner bracket 24. The tab, in combination with several hooks (not shown) on the back of the bracket 24, keeps the cassette 10 in contact with the tensioner bracket and in a clamped shipping position. Alternatively, a hex nut may be used instead of the handle 26. The crankshaft sprocket 18, camshaft sprocket 16, and chain 20 are all in a fixed position that is consistent with the correct timing for the engine timing drive. The tensioner 28 is locked into place by a pin 30 to prevent the tensioner from exerting any additional force on the chain 20. A snubber 32 contacts the other strand of the chain 20. When the tab is rotated out of contact with the tensioner bracket 24, a spring force retracts the tab and its shaft out of the way and allows the tensioner bracket 24 to move away from the crankshaft sprocket 18 vertically downward, aligning the tensioner bracket mounting holes 34 with their respective counterparts in the engine block (not shown).
Chain tensioners are commonly provided in a state of minimum extension to assist in assembly. Pins maintain the tensioner in the minimum extension state, and the pins are removed upon completion of the assembly to the engine. The mechanical tensioner is typically a sub-assembly including a pivoting face, pre-assembled blade springs, a bracket having mounting holes, a perpendicular pivot pin for the pivoting face, and a perpendicular tab on which a compliant face with pre-assembled springs is supported. In the prior art, with the chain wrapped around the sprockets and the tensioner secured in place, the sprockets have too little slack to be mounted on their respective shafts, and the space between the engine block and the chain is too small to allow the tensioner to pass through for assembly.